One of values that represent the property of a fuel is the octane number, which is an index that represents the knock resistance of a fuel. The octane number is 100 for isooctane that has high knock resistance and 0 for n-heptane that has low knock resistance. Furthermore, the octane number of a fuel is a value that corresponds to the % volume of isooctane in a mixed fuel (reference fuel) of isooctane and n-heptane with the same knock resistance. That is, in a case where the knock resistance of a fuel is the same as that of the mixed fuel described above in which the mixing ratio of isooctane is a volume ratio of 50%, the octane number of the fuel is indicated as 50.
Incidentally, there are a variety of types of octane numbers according to the determination technique thereof or the like. Typical examples include the research method octane number that is adopted as an index in countries such as Japan, and the motor method octane number that is adopted as an index in countries such as Germany.
Furthermore, when determining the octane number of an unknown fuel, the octane number is determined by conducting experiments in accordance with the determination techniques described above.
Specifically, in the research method octane number and the motor method octane number, a CFR engine that is specifically designed to measure the octane number is driven by the fuel described above. The octane number is then determined by comparing the pressure fluctuation characteristics of the combustion chamber of a case where the engine is driven by the unknown fuel with a case where the engine is driven by the reference fuel.
The pressure value of the combustion chamber of an internal combustion engine is changed by knocking. The presence of knocking is therefore measured by measuring the pressure value of the combustion chamber. Furthermore, the octane number described above is determined by comparing the measurement timings and the measurement values of the fuel that is the measurement target with those of the reference fuel.